The invention relates generally to the field of image sensors and, more particularly, to such image sensors having a capacitive control gate for eliminating an undesired capacitive effect caused by a reset gate on a floating diffusion and controlling the capacitance of a floating diffusion.
As shown in FIGS. 1 and 2, prior art charge-coupled devices 10 typically include a substrate 20 of a p-type having a buried channel 30 of the n-type for transferring charge packets of electrons. A plurality of gates 40 (two gates for the two-phase device shown in FIG. 1) are connected to the buried channel 30 for controlling charge packet transfer in the buried channel 30. A non-clocked output gate 50 is positioned between one of the gates 40a and a floating diffusion 60 for preventing capacative coupling of the last clocked gate 40a to the floating diffusion 60.
The floating diffusion 60 provides a mechanism for sensing the size of the charge packet for subsequent measurement and the like. A reset transistor 70 provides a mechanism for resetting the voltage level of the floating diffusion after appropriate sampling. A reset drain 80 is adjacent the reset transistor 70 for receiving drained charge from the floating diffusion 60, as is well known in the art.
Referring to FIG. 3, a typical timing sequence of the charge-coupled device 10 is shown. At time T0, the reset gate 70 is clocked high. This turns on the reset transistor 70 and resets the floating diffusion 60 to the reference voltage of the reset drain 80. Then at time T1, the reset gate 70 is clocked low. Capacitive coupling (represented by Creset) between the reset gate 70 and the floating diffusion 60 causes the voltage of floating diffusion 60 to be pushed to a more negative voltage when the reset gate 70 is turned off. The floating diffusion 60 voltage then remains stable and is sampled at time T2. Then at time T3, the gates 40 are clocked which changes their levels and transfers a new charge packet over the output gate 50 and onto the floating diffusion 60. The magnitude of the voltage change at time T3 on the floating diffusion 60 is proportional to the size of the charge packet and the capacitance of the floating diffusion 60. The relationship is given by V=Q/C where V is the voltage change, Q is the size of the charge packet (coulombs), and C is the capacitance of the floating diffusion (farads). The new voltage on the floating diffusion 60 is sampled at time T4 and then the timing cycle is repeated.
Although the currently known and utilized sensor and associated method for transferring charge is satisfactory, it includes drawbacks. The shortcoming of the output structure is the capacitive coupling of the reset gate to the floating diffusion, which causes the voltage glitch between times T0 and T1 on the amplifier input node. This short glitch also restricts the type of electronics that are used to process the output signal of the CCD. Another disadvantage of the prior art is the voltage change of the floating diffusion is fixed to a constant value by the floating diffusion capacitance. Consequently, it is desirable to have a structure, which allows the voltage gain to be changed for some means of altering the floating diffusion capacitance, and which eliminates undesired capacative coupling of the reset gate 70.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, the invention is a charge-coupled device having a plurality of cells for forming the charge-coupled device, each of the cells capable of retaining charge; a transfer mechanism within the charge-coupled device for moving charge through the plurality of cells; an output region for receiving charge moved through the plurality of cells under control of the transfer mechanism; a floating diffusion to receive charge moved across the output region; a reset gate to remove charge from the floating diffusion and reset the floating diffusion to a reference voltage level; and a capacitance control gate adjacent to the floating diffusion for canceling capacitance coupling of the reset gate. A capacitance control gate covers a portion of the floating diffusion and the voltage of the capacitance control gate is adjusted to alter the capacitance of the floating diffusion. The capacitance control gate is clocked opposite that of the reset gate to cancel the capacitive effects of the reset gate.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.
The present invention has the following advantages of producing a smoother waveform going to the CCD amplifier input, which is easier for the electronics of the camera to process. The present invention also provides a means for changing the effective gain of the CCD output.